1. Field of the Invention
This invention pertains to apparatus for monitoring the quality of water, particularly (but not exclusively) the content of organic matter, for example volatile organics and volatile halogenated organics such as trihalomethanes. The desired quality of water depends of course on the intended end-use e.g. for drinking, pharmaceutical manufacture, intravenous uses, feed to high pressure boilers, rinsing of integrated circuits on semi-conducting substrates and the like. One of the quality criteria is the content of organic matter, for example volatile organics including volatile halogenated organics such as trihalomethanes.
Sophisticated and expensive apparatus is available for measuring Total Organic Carbon content, Total Carbon Content, Total Oxygen Demand and the like on a more or less real-time basis (see below). Such apparatus is suitable for a municipal potable water treatment system or a centralized pure water treatment plant but much too expensive for monitoring the quality of water at distributed points of use, for example, potable water in the individual home, ultrapure water at a rinsing station in an integrated circuit production line, dialysis water in a decentralized hemodialysis facility and the like.
At present most municipal potable water treated systems do not thoroughly remove organics from water. Worse, the treatment systems used may produce organics which may be more noxious than those in the water before treatment. For example chlorine used as a disinfectant or to assist in coagulation and/or filtration frequently results in measurable amounts of trihalomethanes (THM's) in the potable water distributed. Ozone is sometimes substituted for at least part of the chlorine in a water treatment system. It does not produce THM's but in the quantities used does not degrade most organics present to carbon dioxide and water but rather to oxygenated organic compounds the physiological effects of which have been little studied. Further inadvertent cross connections between potable water distribution systems and sewage collection systems can introduce organics of household or industrial origin into potable water. Micro-and macro-organisms frequently live in "dead-ends" in potable water systems, avoiding thereby the effects of any residual chlorine in the potable water distribution system and contributing organics from the metabolism of various nutrients in the water. Further, many distribution systems contain plastic pipes from some varieties of which organics can be leached.
The public (becoming educated in the potential harmful effects of organics in distributed municipal potable water), is beginning to install at points of use in their homes devices such as activated carbon and/or reverse osmosis cartridges to remove organics at least partially. Such devices when new and in the absence of defects can remove 90 percent or more of organics from potable water. Some manufacturers include an inexpensive integrating flow meter to indicate on a more or less conservative basis when it is likely that the device should be replaced or rejuvenated. Such meter is of course useless if the device is defective, damaged or improperly installed or if the organic content of the potable water is substantially worse (in composition or concentration) than the manufacturer anticipated. Until now instruments for measuring the organic content of water in the individual home on a more or less real-time basis have been prohibitive in cost (as will be more fully discussed below).
Inexpensive methods are available for measuring organic content of water on a non-real-time basis. These include Chemical Oxygen Demand (boiling a sample of water with chromic and sulfuric acids and determining the quantity of chromic acid consumed) and Biological Oxygen Demand (determining the amount of oxygen used over some days by a bacterial culture injected into a closed sample of water and air. Neither of these are sufficiently sensitive at the concentrations of organics of interest in potable water e.g. 1 ppm or less. Further they require some training and sophistication on the part of the user, not generally available in a household.
A similar situation exists in a production line for integrated circuits on semi-conducting substrates. One would like to know on a real-time basis the quality of water at each of the many points of use. The available instruments for monitoring organics are far too expensive for such purpose. Such instruments are also too expensive for monitoring organics in the dialysis water in dispersed hemodialysis facilities.
It is therefore an objective of this invention to provide an apparatus which is feasible for monitoring the quality of water at points of use on an essentially real-time basis. Other objectives will become apparent from the disclosure and claims below.
2. Description of the Prior Art
Total Oxygen Demand ("TOD") Analyzer: This apparatus uses a furnace operating at about 900.degree. C. and containing for example palladium or platinum wool or gauze. A stream of nitrogen, helium or argon carrier gas containing a low level of oxygen passes through the furnace. The oxygen content down-stream of the furnace is measured electrochemically, e.g. by a doped-zirconium oxide, high temperature oxygen concentration cell. From time to time a precise droplet of water is injected automatically or by manual syringe into the furnace and the depletion of oxygen in the carrier gas measured. The apparatus must be calibrated from time to time against a known standard sample. Such apparatus costs several thousand dollars.
Total Carbon Analyzer ("TCA"): This apparatus also uses typically a 900.degree. C. furnace with noble-metal wool or gauze. The gas passing through the furnace may be air. The carbon dioxide content of the gas downstream of the furnace is measured by a non-dispersive infrared photometer. Again a precise droplet of water is injected from time-to-time into the furnace and the increase in carbon dioxide in the gas stream determined. This apparatus must also be calibrated from time-to-time against known samples.
Alternatively a persulfate salt (or other inorganic peroxide) is injected into a small stream of water which subsequently is highly irradiated with ultraviolet light. The carbon dioxide produced is stripped out of the water and measured by infrared photometry as discussed above.
Both of the above TCA apparatuses also cost several thousand dollars each.
Organic Carbon Analyzer (OCA or TOC): The TCA apparatus discussed above measures not only carbon dioxide from the oxidation of organic carbon compounds but also from bicarbonate ("inorganic carbon") which may be present in the water. OCA devices typically comprise TCA apparatus having a pretreatment section to remove bicarbonate, for example by acidifying and stripping with air. Such pretreatment may also remove volatile organic compounds ("VOC"'s) including THM's. OCA's are inherently more expensive than TCA's.